1. Field of the Invention
This invention generally relates to a bonded substrate and a surface acoustic wave chip, and more particularly, to a substrate in which a lithium tantalate substrate and a sapphire substrate are bonded and a surface acoustic wave chip equipped with the bonded substrate.
2. Description of the Related Art
A surface acoustic wave (hereinafter referred to as SAW) device is produced with a piezoelectric substrate having comb-like electrodes thereon. A high-frequency power is applied to one comb-like electrode to generate surface acoustic waves, and another comb-like electrode converts the surface acoustic waves into high-frequency signals.
The SAW device has a wavelength smaller than that of an electromagnetic wave by 10−5. Therefore, the SAW device can be downsized. The SAW device has a high efficiency in propagation because of a low loss. Additionally, the technique on semiconductor manufacturing processes can be used for the production of the SAW device. This realizes the mass-production and low cost. The SAW device is widely used as a bandpass filter in a communication device such as a mobile telephone.
In recent years, a higher performance has been required for a filter in which the SAW chip is included, according to the high performance of the mobile telephone. One of the requirements for the high performance is the improvement in the temperature stability of the SAW chip. Lithium tantalite (LT) and lithium niobate (LN) are piezoelectric materials having large electromechanical coupling coefficients, which are suitable for realizing the filter characteristics of a wide band. Thus, LT and LN are widely employed in the piezoelectric material of the SAW chip. However, LT and LN have a drawback of inferior temperature stability. The SAW chip made with the above-mentioned piezoelectric materials has a problem in that the passband depends on temperature. In contrast, a quartz crystal, which is also a piezoelectric material of the SAW chip, is superior in the temperature stability, but has a drawback of the small electromechanical coupling coefficient.
As described, as a general tendency of the piezoelectric materials, the piezoelectric materials have two contradictory characteristics. The piezoelectric material having a large electromechanical coupling coefficient is inferior in the temperature stability. In contrast, the piezoelectric material having a small electromechanical coupling coefficient is superior in the temperature stability.
Some techniques have been proposed in order to realize the piezoelectric material having a large electromechanical coupling coefficient and an excellent temperature stability. For example, according to Ohnishi, et al. “Proc. of IEEE Ultrasonic Symposium”, pp. 335–338 (1998) (hereinafter referred to as Document 1), a thin piezoelectric substrate is directly bonded to a thick piezoelectric supporting substrate having a low expansion. Thus, the temperature stability can be improved by suppressing the expansion and contraction caused resulting from the temperature changes. Specifically, the piezoelectric substrate such as LT is mirror finished on both sides thereof. Glass is used for the supporting substrate. The piezoelectric substrate and the supporting substrate are immersed in an aqueous solution into which ammonium hydroxide and hydrogen peroxide solution are mixed to be hydrophilic. Then, the both substrates are rinsed with pure water, and both substrate surfaces are terminated with hydroxyl. When main surfaces of the both substrates are superimposed, moisture is gradually removed and the main surfaces and sub substrates are solidly bonded because of the intermolecular force of hydroxyl, oxygen, and hydrogen (initial bonding). After the initial bonding, the both substrates are heat-treated at least at 100° C. for a few dozens of minutes to a few dozens of hours. The bonded substrate without any residual stress at room temperature is thus obtained (Refer to Document 1 and Japanese Patent Application Publication No. 11-55070 (hereinafter referred to as Document 2)).
The bonding methods disclosed in Document 1 and Document 2, however, need an annealing process at high temperatures. A low expansion material having a small Young's modulus such as glass has to be used for the supporting substrate so that the substrate may not be damaged during annealing. The strain, which is generated by the difference in the thermal expansion coefficients in the bonded substrate, is not transmitted to the piezoelectric substrate sufficiently. This results in an unsatisfactory improvement in the temperature characteristics.
In the case where a single-crystal piezoelectric substrate such as LT is bonded to a single-crystal supporting substrate such as sapphire, the lattice constants of the both substrates are generally different. Even in the case where polycrystalline substrate or ceramics substrate is used for the piezoelectric substrate or the supporting substrate, in most cases, the piezoelectric substrate and the supporting substrate have different lattice constants. If the piezoelectric substrate and the supporting substrate have different lattice constants, a lattice mismatch occurs at the bonded interface and a distortion is thus generated at the bonded interface. Here, this causes problems in that the bonding strength is degraded and the yield ratio of the device becomes lowered.